Special Relativity in 14 Easy (Hyper)steps

14. Why there are magnetic fields

Here's the usual way in which magnetic fields and forces are presented:

  1. current-carrying wires are electrically neutral (there are just as many electrons flowing as there are stationary metal ions)
  2. currents create magnetic fields
  3. magnetic fields exert forces on moving charges:  (where the choice of units may require an additional multiplicative constant to be included in the force law)

A diagram: notice that the moving charge experiences a force directed away from the wire, just as if it were being repelled by a wire carrying a net positive charge. (However, since the stationary charge feels no force, we conclude that the stationary wire is electrically neutral.)

Let's take a micro-look inside the wire. Current flowing to the left corresponds to electrons flowing to the right with, say, speed u. Keep in mind that the wire has as many positive metal ions as negative electrons per meter (since it is uncharged).

The first diagram is from the perspective of observers at rest with respect to the wire, the second from observers who are moving to the right with the same speed as the electrons.

An observer moving to the right will see the wire as having net positive charge since the electron spacing "de-contracts" while the metal ion spacing decreases: there are fewer electrons, but more metal ions per meter of wire.  (This is also true for observers who are not moving at the same speed as the electrons: there's more Lorentz (de)contraction of one sign of charge than the other.)

From the perspective of an observer moving to the right (holding the test charge in the first figure),
the wire will be moving to the left, and will exert an upwards force on the test charge since the wire carries a net positive charge. To this observer, it's not a magnetic force at all, but a Coulomb force due to the presence of an electric field coming from the charged wire.

This is why there are magnetic fields: their effects are a manifestation of the fact that charges exert electrostatic forces on other charges, and that special relativity makes current-carrying wires which are neutral in one frame appear charged in another frame.